Lifting magnet control system



May 29, 1956 LIFTING MAGNET CONTROL SYSTEM Original Filed May 31, 1951 2Sheets-Sheet 1 57%;? 52 Maw/d.

May 29, 1956 J. V. OSWALD LIFTING MAGNET CONTROL SYSTEM Original FiledMay 51, 1951 2 Sheets-Sheet 2 United States Patent F LIFTING MAGNETCUNTRGL SYSTEM Joseph V. Oswald, Chicago, Ill.

Continuation of application Serial No. 229,063, May 31, 1951. Thisapplication April 2, 1953, Serial No. 346,356

Claims. (Cl. 317123) This invention relates to an improved method ofeffecting and controlling the excitation and de-excitation of highlyinductive electrical devices and more particularly relates to a methodof controlling the complete operation cycle of lifting magnets such asare used by the steel industry in the handling of steel and iron in itsvarious forms. It specifically relates, as herein set forth, to liftingmagnet applications wherein the power for energizing the lifting magnetis derived from an individual generator driven by a suitable primemover.

Such a generator is usually operated to serve as a source of constantpotential direct current whereto the magnet is connected to be energizedto pick up and hold the load material and then, through suitable switchmeans, is first disconnected from the generator and then againreconnected in reverse manner to effect a rapid de-energization andfinally complete de-magnetization of the magnet to release the loadmaterial.

As is commonly known by those familiar with the related art, theoperation of highly inductive devices from a constant potential sourceof power is fraught with difiiculties and problems which manifestthemselves in such manner as severe arcing at the contacts used in theswitch means, resulting in burning of the contacts thus shortening thelife thereof, and puncturing of the insulation of the windings of thesaid device due to excessive voltage stresses produced therein by a toorapid collapse of the magnetic flux content, thereby resulting inpremature destruction of the said windings. The high electrical inertiaof some of such devices is also detrimental to satisfactory operation asis also the remnant flux left in the magnetic paths of the device at theend of a free or exponential decline of the said magnetic flux.

With specific reference to the operation of the lifting magnet from aconstant potential power source, the initial connection of the windingsthereof to the said source of energizing power produces no difiicultiesfrom the standpoint of either arcing of contacts or voltage stresses inthe winding. However, due to the high electrical inertia or inductanceof the lifting magnet, the build-up of energizing current through themagnet is retarded considerably so that an appreciable portion of theoperation cycle is invested in that function. This buildup of currentand thus the magnetic flux through the magnet can be accelerated byimpressing a higher than normal initial voltage on the magnet windingsand then reducing said voltage to normal when normal full excitation hasbeen attained in the magnet structure.

In the de-energizing phase of the operation cycle, however, the liftingmagnet, under full excitation, is abruptly disconnected from the powersource and reconnected again in reverse manner as previously described.It is during the transitional functioning of the switch means thatobjectionable arcing of contacts and production of high self-inducedvoltages in the windings occurs. Also, through the switch means, thevoltage of the power source is placed in opposition to the self-inducedvoltage of the magnet to accelerate the decline of the said self-induced2,748,322 Patented May 29, 1956 'ice voltage and the discharge currentdue thereto, but the reverse connection sequence of the said switchmeans generally introduces a limiting resistor into the circuit betweenthe lifting magnet and the power source, the ultimate purpose of whichbeing to limit the value of reverse current flow through the magnetwinding for subsequent final disposition of the remnant magnetic flux.However, the presence of the resistor in the circuit during thedischarge period impedes the flow of discharge current, lengthening thetime required to bring the said discharge current to a zero value andcausing the self-induced voltage in the magnet windings to maintain avalue perhaps several times normal during the discharge period.

In this phase of the operation cycle also, the decline of theself-induced voltage in the magnet windings and the discharge currentdue thereto can be accelerated considerably by raising the opposingpower source voltage substantially above normal during the dischargeperiod of the cycle resulting in appreciably faster unloading ofmaterial from the magnet.

In the final or de-magnetizing step of operation the remnant flux in themagnet structure is disposed of by permitting a small value of currentto build up through the magnet windings in a reverse direction thusreducing the remnant flux to zero, or, more generally, causing the valueof the remnant flux to be passed through zero to a small value ofopposite polarity at which time the fine load materal will be releasedas the remnant flux passes through zero or when the reverse excitationis finally removed.

Such lifting magnet installations wherein an individual generator isused as a power source are generally susceptible to conversion from theconstant potential system to the well known variable voltage system ofoperation by the addition of a suitable source of separate excitationfor the exciting field of the generator and, by the further substitutionof the switch means herein described for the constant potential switchmeans previously used, to provide a lifting magnet control methodsuperior to methods now in general use.

This invention, therefore, proposes to apply the variable voltageprinciple to the operation of the lifting magnet with the object in viewof providing a method of lifting magnet control wherein:

(1) The initial voltage applied to the magnet windings for energizingthe magnet is substantially above the normal rated voltage of the magnetto thus accelerate the build-up of current flow through and consequentlythe flux content within the magnet to its normal value and wherein thesaid initial voltage declines to normal value as the excitation of themagnet attains its normal maximum value.

(2) The inherent volt-ampere characteristics of the type of generatoremployed may provide an appreciable regulatory function, so that thevoltamperes of power supplied to the magnet when the windings are coldand therefore of lower resistance, is lower than that supplied when thewindings have reached normal or even abnormal operating temperature.Thus, the attractive power of the magnet tends to remain more uniformduring the steady state phase of the cycle over a protracted period ofmagnet operation.

(3) The de-energizing function of the control provides a voltagesubstantially above normal in opposition to the self-induced voltage ofthe magnet to accelerate the decline of the magnet discharge current toa zero value to expedite unloading of material from the said magnet.

(4) The de-energizing function of the control also provides a means ofinterrupting or commutating the circuit between the generator and themagnet at a time of minimum current flow and a minimum of potential aminimum of arcing at contacts and with a minimum of current and voltagedisturbance within the windings of the generator and magnet.

(5) The flow of magnet discharge current occurring during thede-energizing phase of the operation cycle takes place through a circuitof negligibly low ohmic resistance, so that the self-induced voltage ofthe magnet does not exceed the opposing generator voltage by a valuegreater than that which is propo 'ional to the ampere-impedance dropthrough the components of the circuit between the magnet and generator.

(6) The de-magnetizing function of the control disposes of the remnantmagnetic flux by permitting the build-up of a small value of reversecurrent through themagnet windings after the discharge current of themagnet has been reduced to a zero value and after a predetermined timeand, upon attainment of certain electromagnetic conditions within thelifting magnet, automatically acts to remove the said reverse current.

The invention comprises other objects, advantages and capabilities whichwill become apparent upon perusal of the attached drawings andspecification, which show a preferred embodiment of the said invention.Howeverfsuch preferred embodiment is not intended to precludemodification of the invention and therefore it is to be understood anysuch modification need not necessarily be regarded as a departure fromthe spirit of the invention as herein set forth.

In the interest of subsequent understanding of the nature of theinvention, attention is directed to the drawings of which Fig. 1represents a schematic diagram of the complete system;

Fig. 2 is a line diagram of the power circuit or, more specifically,that portion of the system involved in the direct conduction andcommutation of the energizing and discharge currents which flow betweenthe generator armature member and the lifting magnet windings inthe'course of operation;

Fig. '3 is a line diagram of the control circuit or that portion of thesystem which controls the actuating function of the various switch meansand also the application, reversible manipulation and final removal ofexcitation to and from the main exciting field winding of the generator;

Fig. 4 is a graphic illustration of the complete operation cycle of thecontrol system wherein the variations of voltage, current andampere-turns of field excitation occurring thruout the operation cycleare presented in synchronous sequence. Their respective ordinate valuesas shown, however, do not necessarily bear a fixed relationship to eachother but are merely intended to illustrate their approximatemagnitudinal variation thruou thecycle. To that end, the reference scaleon the left of the graph serves as a comparathle means.

For a general description of the major components of the inventionreference is made to Fig. l of the drawing, wherein a lifting magnet itis to be energized by current generated in the armature member of agenerator comprising said armature member ill, a separately excited mainfield i2 and an auxiliary series field winding 13. The generator chosenfor this preferred embodiment, comprising members 11 i2.i3 is of thetype known to the art as a diii rentially compounded machine wherein,under normal operating conditions, the ampere-turns of the series, fieldwi 13 are opposed to the ampere-turns of the main field winding 12andthe resulting excitation which is applied to the field structure of thegenerator is a net excitation of a value which is equal to thedifference of the two values of ampere-turns. Thus, such a generatorwill have'a drooping voltage characteristic from no load to full'loa'dcondition and the ratio of no load voltage to full load voltage isdetermined by the relative values of ampereturns developed by main fieldwinding 12 and series field winding 13.

Excitation to the separately excited main field winding 12 is obtainedfrom a separate source of direct current (not shown) such as an exciter,rectifier or battery, etc., but which is herein represented by thecharacters E1 and E2.

Excitation to main field winding 12 is applied through the medium of afirst switch A, which comprises a plurality of contacts as shown'and isactuated either manually or, as shown in this preferred embodiment,electroresponsively by a magnet 15 when said magnet 15 is energized fromseparate source E1 and E2 by closing a pilot switch 14. Contacts 1, 2and 3 of first switch A are normally held open and contacts 4 and 5 arenormally held closed by the biasing action of a tension spring 16.

Separately excited rnain field winding 12 is connected into a networkcomprising winding 12 and contacts 1, 2, 4 and 5 as shown whereby mainfield'winding 1-2 may, in the course of operation, be reversiblyconnected to the separate excitation source E1 and E2.

Contact 3 of first switch A serves as a pilot contact for energizing thewinding I18 of a second switch B from separate excitation source E1 andE2.

Second switch B may be of the type known to the art as a clapper typecontactor, which may be provided with two windings as shown. to beexcited by'separate source E1 and E2 and may consist of many turns ofsmall section conductor, while winding 19 is adapted to be excited bythe relatively heavy currents flowing between generator 11 and liftingmagnet id; and thus may consist of comparatively few turns ofcomparatively large section conductor.

The-electrical proportioning of windings 18 and 19 is based on theobservation that the number of ampereturns of magnetomotive forcerequired to close and'seal the armature and contacts of such a switch asrepresented by the second switch B may be an average of ten times thenumber of ampere-turns of magnetomotive force required to hold such aswitch closed with an effectiveness just short of the release point.Thus winding 18 may be'propo'rtioned to exert, when energized fromsource E1 and E2, sufficient attractive force to close and seal thearmature'and contact 6 of second switch B. Winding 19 may beproportioned to exert, with a given value of current flowingtherethrough and winding 18 tie-energized, the minimum necessaryattractive force to hold the armature and contact 6 closed while thecurrent flow therethrough is equal to or in excess of the abovementioned critical value. Thus, switch B will close upon energizingwinding 18, flow of substantial current through co-acting winding 1%will augment the attractive force of winding 18, holding switch B closedmore positively, switch 3 will remain closed upon de-energizing winding18 if the current fiow through winding 19 is equal to or in excess ofthe aforementioned critical value, and decline of the current flowthrough winding 19* beiow the said critical vaiue will result in theopening of second switch B.

A third switch C is of the type referred to in the art as a magnetictime delay switch, wherein the variations in magneticfiux content of theactuating magnet are retarded by the action of a lag coil 22 consistingof a copper sleeve or other winding shortcircuited upon itselfelectrically, thus resulting in a delay in the response of third switchC to the variations of the ampere-turn force of its actuating windingThe structural and functional characteristics of third switch C are suchthat a substantial flow of current through winding 29 in one directionserves to attract the armature of said third switch C and'close'contacts7 and -53. The retarding action of lag coil 20 delays the closingsomewhat, but in the sequence of operation 'ofthe control system hereinde- Winding 18 is adapted scribed such delay is of negligibleconsequence. Subsequent decline of the current flow through winding 20to a zero value does not serve to open third switch C but, due to theaction of the lag coil 22 and the residual magnetic flux remaining inthe frame of the actuating magnet of third switch C, it is required thata small value of current of some time duration be passed through winding20 in a reverse direction in order to eliminate the said residual fluxagainst the retarding action of lag coil 22 to thus release the armatureof third switch C and open contacts 7 and 8.

Proceeding to the description of the operation of the control system,reference is again made to the Figs. 1, 2 and 3 of the appendeddrawings.

Energizing-Assuming that power of suitable nature and value is availableat El and E2, and that armature 11 of the generator is rotating atrequired speed, the operation cycle is initiated by the closing of pilotswitch 14, thus energizing winding 15 of first switch A from source E1and E2, and closing contacts 1, 2 and 3 and opening contacts 4 and 5against the action of biasing spring 16. Closure of contacts 1 and 2applies excitation to separately excited main field winding 12 fromsource E1 and E2 in what may be assumed for purposes of description asthe positive direction, as distinguished from the reverse or negativedirection to be encountered later in the operation cycle.

Simultaneous closure of contact 3 energizes winding 18 of second switchB, closing contact 6 to establish the circuit between generator armature11 and lifting magnet 10. Reference to Fig. 2 of the drawing shows thatat this stage of the operation cycle, armature 11, series field winding13, lifting magnet winding 15), winding 20 of third switch C, the nowclosed contact 6 of second switch B, and winding U of said second switchB are all connected serially into a power loop (so called in the art)and are acted upon simultaneously by the ensuing flow of currentoriginating in armature 11. Flow of said ensuing current through winding19 of second switch B serves to augment the attractive force of winding18 of said second switch B to hold contact 6 closed.

Flow of said current through winding 20 of third switch C attracts thearmature of said switch C after a slight delay, due to the retardingaction of lag coil 22, to close contacts 7 and 8. Closure of contact 7establishes a subsidiary circuit through a resistor 21, the function ofwhich will become clear in the final phase of the operation cycle.Closure of contact 8 establishes a portion of the circuit for thesubsequent reverse or negative excitation of separately excited mainfield winding 12 (as distinguished from the previously mentioned forwardor posi tive direction of excitation), said circuit for negativeexcitation being presently maintained open by the now open contacts 4and S. The lifting magnet is now energised and ready to be applied tothe load.

Steady state.The so called steady state of the operation cycle is thatperiod immediately following attainment of full energization of themagnet and immediately preceding the de-energizing phase of the cycle.During the steady state period the magnet is applied to the loadmaterial, which is then picked up and transferred to the point ofdeposit.

Abrupt application of the energized magnet to the load material producesa momentary dip in the current flow to the magnet. It should be notedthat the switch means does not respond to such current dips, even ifextreme in magnitude, since second switch B is held closed by winding18, which is energized from source E1 and E2 and, due to thepredominance of the attractive force of winding 18 over that of winding19, it would be necessary for the current flow to swing considerably tothe reverse or negative direction to cause a neutralization of winding18 by winding 19, a condition which is not normally encountered inpractice. Such a reversal in current flow, if of consequentialmagnitudes and duration, might conceiv- 6 ably cause third switch C toopen, but opening of contacts 7 and 8 at this time is of no consequenceand upon resumption of normal current flow, switch C would again close.

Loss of all or a substantial part of the load in transit such as a largecasting, etc. produces a momentary rise in the current flow to themagnet and a rise in current flow would obviously tend to increase theattractive force of windings 19 and 20 of switches B and C respectively,thus precluding their opening under such a condition.

De-energizing.With the load material picked up and transferred, thede-energization of the magnet and con sequent release of the loadmaterial is initiated by the opening of pilot switch 14, de-energizingwinding 15 of first switch A, opening contacts 1, 2 and 3 and closingcontacts 4 and 5 by the action of biasing spring 16. Opening contact 3de-energized winding 18 of second switch B, but due to the presence ofcurrent in winding 19, switch 13 and consequently contact 6 remainclosed.

Opening of contacts 1 and 2 and closing of contacts 4 and 5 of firstswitch A transfers separately excited main field winding 12 from aforward or positive excitation to a reverse or negative excitation,resulting in an almost abrupt reversal of the generator voltage. Thisreversal of generator voltage initiates a reduction of current flow tothe magnet, with the result that a self-induced voltage; arises in themagnet winding in opposition to the now reversed generator voltage.During the de-energizing, stage of the cycle, this self-induced voltageexceeds the opposed generator voltage to the extent that the currentflow in the power loop continues in its forward direction, though nowunder the impetus of the self-induced voltage of the magnet windingsinstead of the voltage induced in armature 11 of the generator. The sumtotal of this is that during the de-energization process the magnetwindings become, in effect a generator and the generator proper becomesa motor against whose counter-electromotive force the magnet windingsdischarge the current of selfinduction.

This fiow of discharge current in the original forward direction actsupon winding 19 of second switch B to keep the power loop closed atcontact 6 and also acts upon winding 20 of third switch C to keepcontacts 7 and 8 closed for subsequent functioning at the end of thecycle.

This flow of discharge current against the opposing generator voltagecontinues in a progressively declining manner until the critical valueis reached which will cause winding 19 of second switch B to release thearmature of said switch B and open contact 6 to commutate (or alter) thepower loop to a new condition of electrical composition.

For a theoretical illustration, it may be assumed that second switch Bis adapted to open at a zero current flow through winding 19. This valueof zero current will obtain when the self-induced voltage of the magnetwindings is exactly equal to the opposing generator voltage. Under suchan ideal condition contact 6 would open with a state of zero-volts andzero-amperes existing at its contact surfaces at the instant of opening,and an arc would be non-existent.

Actually, such a response is not practical of attainment, but any moreor less minor deviation therefrom under practical conditions does notdetract from the effectiveness of the arrangement materially. If switchB is adapted to open at a discharge current value equal to approximately2% of the normal energizing current of the magnet, the are at contact 6,is almost imperceptible. This has been determined to be due in somemeasure to the inherent mechanical lag in the switch mechanism and sincethe discharge current is in a state of rapid decline at the time ofopening of contact 6, a small amount of mechanical lag tends to delaythe instant of opening toward the zero-current point.

De-magnetizing.-When the self-induced voltage in the magnet windings andthe counter-voltage of the motorized generator strike abalance, thedischarge current has declined to a zero value and contact 6, havingopened, in-

troduces'into the power loop the subsdiaiy circuit containbalance andzero discharge current, theself-indu'ced volt-' age'of the magnetwindings declines below the counter-' voltage of the motorizedgenerator, whereupon the gen-' erator again resumes its normal functionas such and initiates a build-up of current through the power loop in areverse direction. This build-up of reverse current is paced by thestill declining self-induced voltage in the magnet windings and reachesits final value upon attainment of a slight degree of reversemagnetization of the lifting magnet structure. The final steady statevalue of the reverse current is determined by the combined resistance ofthe lifting magnet and resistor 21 and the generator voltage obtainingat that stage of the operation cycle.

The build-up of the reverse current to its final value also acts uponwinding of third switch C to eliminate the residual flux in theactuating magnet frame of said switch C and this process is opposed andretarded by the lag action of winding 22. After a time interval, theresidual flux in switch C is reduced to the extent that the armaturethereof is released, opening contacts 7 and 8. Opening of contact 7opens the power loop, finally disconnecting the magnet from thegenerator to remove the reverse current from the lifting magnet, and theopening of contact 8 opens the reverse excitation sequence of thenetwork involving separately excited main field winding 12, thusremoving excitation from the generator and causing the generator voltageto fall to a zero value, completing the operation cycle.

The system may be made adaptable to virtually all sizes of magnetsfalling within the capacity of the generator by providing an adjustableresistor 21 and a multitapped winding 20 for third switch C to permit ofobtaining a suitable time interval for the demagnetizing function.

The foregoing description of the operation cycle served to illustratethe operation of the switch means A-B-C and no account was taken of thevoltage and current transients occurring within the power loop duringthe said cycle.

Reviewing the operation cycle from the standpoint of the graphicalrepresentation of Fig. 4, which is based on the characteristics of thetype of generator used in the herein disclosed control system,identification of the various curves shown discloses that:

The curve A denotes a constant value of excitation applied to separatelyexcited field winding 12 from source E1 and E2. For the sake of clarity,this valueis shown as being equal in both the forward or positive andthe reverse or negative directions as comparison by suitable A meansWill show.

The curve D represents the value of ampere-turns of excitation developedby separately excited winding 12 for between generator armature 11 andlifting magnet wind ing 10.

The curve B represents the resultant or net excitation impressed on thegenerator field structure as a result of the combined action of theampere-turns of windings 12 and 13, curves D and C.

The curve E represents the voltage outputof the 'gen-' erator as aresult of the excitation represented by curve B' i and is proportionalto said curve B'throughou't the operation cycle.

The curve F represents the variation in magnitude and polarity(direction) of the current flow between generator armature 11 andlifting magnet winding 10.

The curve G represents the value of voltage appearing at the liftingmagnet terminals in the final or de-magnetizing phase of the cycle.

The curve H is an approximation of the magnitude of the self-inducedvoltage occurring in the magnet windings during the deenergizing phaseof the operation cycle.

Energizing-Closing of contacts 123-6 and opening of contacts i5 appliesexcitation to separately excited field winding 12 and also closes thepower loop as described. Excitation of the ampere-turn value of curve Dis applied to field winding 12 and rises to its full valve almostimmediately because of the relatively low electrical inertia of such awinding. The resulting generator voltage initiates the build-up ofcurrent flow to the magnet (curve F) but, due to the high electricalinertia of the magnet, the build-up of current therethrough andconsequently the rise of the counter-acting the magnet to energize itstarts from an above normal value and recedes to normal as the magnetattains full energization.

Steady state.ln the steady state of energization, the positive mainfield ampere-turns (curve D) combined with the negative series fieldampere-turns (curve C) produce a positive net field excitation (curve B)of a value required to produce the normal positive generator voltage(curve E) in turn required to pass the normal current (curve F) throughthe lifting magnet.

In the practice of lifting magnet operation, the energization of themagnet is generally done prior to applying the magnet to the load it isthen customary to plunge the magnet into the load material, presumablyfor more efficient loading of the magnet. in plunging the magnet into adense mass of load material, the abrupt-closing the working gap of themagnet results in an equally abrupt reduction or" the rel ctance of themagnetic paths of the now combined magnet frame and load material,requiring an additional braid-up of magnetic flux therein.

This additional build-up of magnetic flux causes a voltage se in themagnet windings in oppoot' self-induction to ri sition to the generatorvoltage, causing a dip in the current drawn by the magnet. The currenthas to then recover to its normal value while building up the new valueof magnetic iiux required to hold the load material.

Therefore, in the steady state portion of the operation cycle (thoughnot so illustrated in Fig. 4), any temporary reduction of the magnetcurrent (curve F) caused by the above described practice results in areduction of opposing ampere-turns of series field winding 13 (curve C),

with the further result that the net field excitation (curve B) andconsequently the generator voltage (curve E) both rise to speed therecovery of the magnet current'to normal, said net field excitation andgenerator voltage returning to normal upon establishment of the newvalue of magnetic flux in the magnet and load material.

When a loss of all or a substantial part of the load occurs in transit,the reluctance of the magnetic paths is abruptly increased, causing adecrease in the magnetic flux content of the magnet, and a self-inducedvoltage again rises in the magnet windings but in this instance itcoincides with the generator voltage and a rise in magnet currentensues. The rise in magnet current (curve F) causes an increase inopposing series field ampere turns (curve C) resulting in a decrease inthe net field As the current fiow to the mag-- excitation (curve B),lowering the generator voltage (curve B) to thus reduce the totalvoltage in the power loop tending to maintain this abnormal currentflow. As the magnet flux becomes stabilized at its new value, thevoltage (curve E) and the current (curve F) again return to normal.Thus, the generator herein used exerts a stabilizing influence upon thesystem during the steady state period of the operation cycle.

De-energizing.pening of contacts 123 and closing of contacts 45 resultsin reversal of excitation to separately excited winding 12 and thegenerator voltage (curve E) abruptly reverses polarity, initiating thedecline of the current (curve F). Continued flow of the current (curveF) is at a declining rate and is compelled by the self-induced voltage(curve H) in the magnet windings, which voltage is in opposition to andgreater than the generator voltage (curve E) by a value necessary tosend the said current (now referred to as the discharge current) throughthe power loop in the original forward or positive direction. Thus,during the de-energizing period of the cycle, the differentiallycompounded generator now becomes a cumulatively compounded motor whoseback-voltage or counter-E. M. F., as it is referred to in the art, isthe product of the now co-acting ampere-turns of winding 12 (curve D)and winding 13 (curve C) and the said back-voltage may rise to itsmaximum or saturation value.

Decline of the self-induced voltage and consequent discharge of magnetcurrent against the above normal opposing voltage of the motorizedgenerator proceeds to the point where the tension of the magnetic fieldof the lifting magnet has been relieved to the extent that the value ofself-induced voltage in the magnet windings will be equal and stillopposed to the voltage of the still more or less motorized generator atwhich time the discharge current will have declined to a near zero value(point 0). At this time, contact 6 will open to introduce resistor 21into the power loop in preparation for the reversal of the current(curve F) for subsequent demagnetization of the lifting magnet.

Thus, during the de-energizing phase of the operation cycle the voltageapplied to the magnet to accelerate the decline of the self-inducedvoltage and the discharge current due thereto is again of higher thannormal value.

De-magnezfizing-Contact 6 having opened at approximately the instant ofvoltage balance and zero discharge current in the power loop, point 0,the generator now resumes its function as such and passes a reversecurrent through the magnet and the third switch C to complete the cycleof operation as previously described.

During the de-magnetizing stage of the cycle the generator voltage willagain be above normal since the again opposing ampere-turns of winding13 (curve C) are now of smaller value and the net field excitation(curve B) is above normal. Upon attainment of the steady state in thede-magnetizing function, the voltage at the generator coincides withcurve E but the voltage at the magnet terminals falls to the valuerepresented by curve G, the difference between the two representing thevoltage drop across resistor 21.

Thus, upon final disconnection from the generator, the voltage at themagnet terminals and the value of current flowing therethrough are ofsuch low order as to be easily handled by comparatively small switchmeans and the voltage transients occurring at that time are ofnegligible consequence.

With reference to the regulatory function of the type of generatoremployed in this control system, it is commonly known that the powerrating of a lifting magnet, as noted on the nameplate thereof, isgenerally that power required to fully energize the magnet at its normaloperating temperature. A cold magnet will have a lower ohmic resistancethan a hot magnet and will, therefore, require a lower voltage at itsterminals to pass the rated current therethrough.

The type of generator used in the control system herein disclosed iscapable, to some extent, of compensating for temperature changes, bothin its own windings and those of the lifting magnet. The degree ofcompensation is dependent upon the volt-ampere characteristics of thegenerator and that is in turn determined by the propertioning ofwindings 12 and 13 and the degree of magnetization attained in themagnetic paths of the machine.

This feature is, however, not essential to the operation of this controlsystem, the chief objective of the use of such a generator being theattainment of the above normal voltages during the transitional stagesof the operation cycle, namely energizing, de-energizing anddemagnetizing, resulting in accelerated functioning of the system and acorrespondingly rapid response of the magnet in the handling of loadmaterial.

This is a continuing application of my application Serial No. 229,063,filed May 31, 1951.

The invention having thus been disclosed and described, what is claimedas new and desired to be covered by Letters Patent is:

1. In a lifting magnet control system, a lifting magnet having windingsenergized from a generator driven by a suitable prime mover, saidgenerator having a separately excited main field winding; a separatesource of electrical power suitable for the excitation of saidseparately excited main field winding; a first switch means operable toreversibly connect said separately excited main field winding to saidseparate source of electrical power to effect a reversal of the polarityof the potential of said generator and consequently etfect a stoppage orreversal of the direction of the current flow between said generator andsaid lifting magnet windings, a second switch means electro-responsiveto the current fiow between said generator and said lifting magnet andoperable to close upon substantialcurrent fiow from said generator tosaid lifting magnet in one direction, said second switch operable toopen after a predetermined time-current interval of current flow fromsaid generator to said lifting magnet windings in a reverse direction, afirst winding on the actuating magnet of said second switch adapted tobe excited by the current flow between said generator and said liftingmagnet windings, a second winding on said second switch comprising acontinuous winding of electrically conductive material short-circuitedupon itself and placed in mutually inductive relation to said firstwinding, such arrangement effecting a delay in the response of saidsecond switch to de-energization of said first winding; contact means onsaid second switch connected into the reverse sequence connection ofsaid first switch, opening of said contact means adapted to disconnectsaid separately excited main field winding from said separate source ofelectrical power.

2. In a lifting magnet control system, a lifting magnet having windingsenergized from a generator driven by a suitable prime mover, saidgenerator having a separately excited main field winding; a separatesource of electrical power for excitation of said separately excitedmain field winding and other components of said control system; a firstswitch operable to reversibly connect said separately excited main fieldwinding to said separate source of excitation to effect a reversal ofthe polarity of the potential of said generator and consequently toeffect a stoppage or reversal of the direction of the current flowbetween said generator and said lifting magnet windings; a second switchadapted to establish the circuit for, and commutate the current flowbetween said generator and said lifting magnet windings, said secondswitch electroresponsively operable to close upon excitation of aclosing winding provided on the actuating magnet thereof, said secondswitch electro-responsively operable to open upon the de-energization ofsaid closing winding and a decline of current flow below a predeterminedvalue through a holding winding provided on the actuating magnet of saidsecondswitcn'said closing winding on said second switch adapted to beexcited from the said separate source of electrical power, said holdingwinding adapted to be excited by the current flow between said generatorand said lift magnet windings; and a resistance means for limiting thecurrent flow between said generator and said lifting magnet windings toa value suitable for disposition of the remnant flux in the liftingmagnet structure.

3. In a lifting magnet control system, a lifting magnet having windingsoperated from a generator driven by a suitable prime mover,said'generator having a separately excited main field winding, saidgenerator having an auxiliary main field winding adapted to be excitedby the current flow between said generator and said lifting magnet, saidauxiliary main field winding being connected to counteract saidseparately excited main field winding in course of normal operation ofsaid generator; :1 separate source of excitation for said separatelyexcited main field winding and other components of said control system;a first switch operable to reversibly connect said separately excitedmain field winding to said separate source of excitation to effect areversal of the polarity of the potential of said generator andconsequently effect a stoppage or reversal of the direction of thecurrent flow between said generator and said lifting magnet; 21 secondswitch adapted to establish a circuit for, and to commutate the currentflow between said generator and said lifting magnet windings; saidsecond switch electro-responsively operable to close upon excitation ofa closing winding provided on the actuating magnet thereof; said secondswitch electro-responsively operable to open upon de-energization ofsaid closin winding and decline of current flow below a predeterminedvalue through a holding winding on the actuating magnet of said secondswitch; said closing winding adapted to be excited from said separateexcitation source, said holding winding adapted for excitation by thecurrent flow between said generator and lifting magnet windings; and aresistance means for limiting the current flow between said generatorand lifting magnet windings to a value suitable for disposition of theremnant flux in the lifting magnet structure.

4. In a lifting magnet control system, a magnet having windings operatedfrom a generator driven by a suitable prime mover, said generator havinga'separately excited main field, said generator having an auxiliary mainfield winding adapted to be excited by the current flow between saidgenerator and said lifting magnet windings, said auxiliary main fieldwinding being connected to counteract said separately excited main fieldWinding in the course of normal functioning of said generator; aseparate source of excitation for said separately excited main fieldwinding and other components of said control system; a first switchelectro-responsively operable to reversibly connect said separatelyexcited main field winding to said separate source of excitation toreverse the potential polarity of said generator to effect a stoppageterminedvalue through a holding winding-thereupon to open said actuatingmagnet of said second switch; a resistance for limiting the current flowbetween said generator and said lifting magnet to a value suitable fordisposition of the remnant flux in the lifting magnet structure; a thirdswitch electro-responsive to the current flow between said generator andsaid lifting magnet and operable to dose upon substantial current flowfrom generator to said lifting magnet in one direction, said switchoperable to open after a predetermined timecurrent interval of currentflow from generator to said lifting magnet in a reverse direction, afirst Winding on the actuating magnet of said third switch adapted to beexcited by the current flow between said generator and said liftingmagnet windings, a second winding on said third switch comprising acontinuous winding of electrically conductive material short-circuitedupon itself and placed in mutually inductive relation to said firstwinding, such arrangement effecting a delay in the response of saidthird switch mechanism to de-energization of said first winding; acontact on said third switch connected into the reverse sequenceconnection of said first switch, opening of said contact adapted toeffect the removal of excitation from said separately excited main fieldwinding.

5. In a lifting magnet control system, the combination of the lifingmagnet windings operated from a generator driven by a prime mover, saidgenerator having 21. separately excited mainfield winding, saidgenerator having a series field winding connected to counteract saidseparately excited main field winding in the course of normal generatorfunction; a separate source of excitation for said separately excitedmain field winding and other control systemv components; anelectro-responsive first switch energized from said separate source ofexcitation and operable to reversibly connect said separately excitedmainfieldwinding to said separate. source of excitation; anelectro-responsive second switch having contacts to initially establishand subsequently commutate the circuit between said generator andlifting magnet; said second switch having two actuating windings, one ofwhich is adapted to be excited by said separate source of excitation andthe-other adapted to be excited by the current flow between saidgenerator and lifting magnet; an electro-responsive third switch havingan actuating winding adapted to be excited by the current flow betweensaid generator and lifting magnet, said third switch having ashort-circuited winding placed in mutually inductive relation tosaidactuating winding to effect a delay in the response of saidthirdswitch to the rise or decline of current in said actuating winding; saidthird switch having contacts adapted to initially establish andsubsequently interrupt circuits controlling excitation to saidseparately excited main field winding, said third switch havingcontacts. adapted to initially establish a subsidiary circuit througharesistance to limit the value of de-magnetizing current for disposal ofthe remnant flux within the lifting-magnetstructure and finally tointerrupt the circuit between said generator and liftingmagnet.

References Cited in the file of this patent UNITED STATES PATENTS2,206,823 Wertz July 2, 1940 2,257,361 Yorkey Sept. 30, 1941 2,287,745Morawetz June 23, 1942 2,390,377 Lillquist Dec. 4, 1945

